Summary of Significant Accounting Policies - Summary of Derivative Financial Instruments (Details) - Fair Value, Measurements, Recurring - Black Scholes Model - Derivative Financial Instruments - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Liabilities:
Fair value, liabilities	$ 5,743	$ 13,769
Level 3
Liabilities:
Fair value, liabilities	$ 5,743	$ 13,769